Chromium plating



, Patented Jan. 9,1934 I l h UNITED. STATES PATENT OFFICE CHROMIUM PLATING George E. Barber, Detroit, Mich, assignorto The Timken-DetroitAxle Company, Detroit, Mich, a corporation of Ohio No Drawing. Application October 3, 1931 Serial No. 566,791

9 Claims. (Cl. 204--1) The principal objects of the present invention When the process of electro-deposition is carare to devise a process of chromium plating that ried out under the conditions specified, the operais cheaper, more facile and more rapid than proction is rapid, facile and economical, and the reesses heretofore practiced and which will prosulting coating is very bright, adherent, shock 5 duce a bright and more adherent coating. resistant and of uniform thickness; in fact, my 60 Heretofore it has been generally considered process produces much better'coatings up to .06 that an electrolyte bath would be very unsatisfacinch thick than are produced by other processes tory if the ratio of chromic acid to sulphuric acid now in use; and it does so in much shorter time (CrOa:SO4) therein is smaller than 1 to 50 or 1 than is required by such other processes.

to 40. I have found, however, that under certain In carrying out my process, it is practicable 65 conditions hereinafter set forth, a bath with such to vary from the specific strength, ratios, temlow ratio will produce an unusually bright coating perature and composition of the bath above set which is not only very adherent but also very forth, and also the current density; Thus, the resistant to shock. temperature and the amperage may be varied According to the present process, an electrolyte considerably in accordance with knowledge famil- 70 bath is made comprising a chromic acid solution iar in the art. Likewi the ratio of chromic containing sulphate. A suitable bath comprises acid to sulphate y b V ed through a range 240 grams of chromic acid (CrOa) per liter and of ten to forty parts of chromic acid to one part 13 grams of sulphuric acid ($04) or equivalent of sulp fl The ratio of trivalent iu to soluble catalytic salt per liter. In this example, hexavalent chromium also admits of variation 75 the ratio is about 18.5 parts of chromic acid but should not be less than one part of trivalent (CIOs) to one part of sulphuric acid (S04). Care chromium to ten parts of hexavalent chromium is exercised to avoid the presence of iron in the and good results are attainable when the ratio bath. For this purpose, the bath is used in a lead is one to one.

lined tank and preferably lead anodes are used, What I claim is: v 80 although small steel anodes have been used suc- 1. In the process of electroplating with chrocessfully. Preferably the density of the current mium, the improvement which comprises mainis approximately 900 amperes per square foot, taining a ratio of chromic acid (CrOa) to sulalthough it is practicable to operate with a conphuric acid (S04) in the bath less than 40 to 1,

siderably lower amperage, say as low as 5'75 to 600 the bath containing trivalent chromium and 85 amperes per square foot. For the sake of rapidity hexavalent chromium in the proportion of not less it is preferable to keep the current density above than 1 to 10. 750 amperes per square foot. The temperature 2. In the process of electroplating with chroof the bath is about 130 to 135 Fahrenheit. mium, the improvement which comprises main- Otherwise the process is carried out in accordtaining a ratio of chromic acid (CrO3) to sulance with the ordinary practice of electro-deposiphuric acid (S04) in the bath of from 10 to 20 tion of chromium. parts by weight of chromic acid (CrOa) to 1 part The chromium content of the bath is partly by weight of sulphate (S04), such chromic acid trivalent chromium (Cr and partly hexavalent comprising trivalent chromium and hexavalent chromium (Cr preferably in the approximate chromium in 'a ratio of between 1 and 10 parts 95 ratio of one part of trivalent chromium to five, by weight of hexavalent chromium to 1 part by of hexavalent chromium (1C1 Z5Cr The weight of trivalent chromium. content of trivalent chromium in the bath can 3. In the process of electroplating with chrobe increased by the addition of chromic carbonate mlum, the improvement which comprises mainor other soluble trivalent chromium salt or by taining a ratio of chromic acid (CrOa) to sulageing, that is, by allowing the bath to work a phuric acid (S04) in the bath less than 40 to 1, sufficient time to become active. The operation the bath being kept at a temperature of approxiof ageing is a progressive change of hexavalent mately 130 to 135 Fahrenheit and containingtrlchromium to trivalent chromium; and it may valent chromium and hexavalent chromium in produce satisfactory results in a few hours or it th pr p rtion of not less thanl to 10.

may require several days, its purpose being to 4. The process of electroplating with chroproduce the desired ratio of trivalent chromium mium, which comprises maintaining a bath to hexavalent chromium, such ratio being readily wherein chromic acid (CrOs). and sulphuric acid ascertainable by suitable test or chemical analysis (S04) are present in a ratio of less than 40 parts of the bath. by weight of chromic acid (CrOs) to one part by weight of sulphuric acid (S04) with the chromium present in the proportion of about 1 part by weight of trivalent chromium to from one to ten parts by weight of hexavalent chromium, the temperature of the bath being approximately 130 to 135 Fahrenheit, and the current density being in the neighborhood of 600 to 900 amperes per square foot.

5. The process of electroplating with chro mium, which comprises maintaining a bath substantially free from iron and wherein chromic acid (CrOa) and sulphuric acid (S04) are present in a ratio of less than 40 parts by weight of chromic acid (010:) to one part by weight of sulphuric acid (S04) with the chromium present in the proportion of about one part by weight of trivalent chromium to from one to ten parts by weight of hexavalent chromium, the temperature of the bath being approximately 130 to 135 Fahrenheit, and the current density being in the neighborhood of 600 to 900 amperes persquare I foot.

6. Theprocess of electroplating with chromium, which comprises maintaining a bath wherein chromic acid (CrOa) to sulphuric acid- (SO4) are present in a ratio of about 18.5 paits by weight of chromic acid (CrO3) to one part by Weight of sulphuric acid (S04) with the chromium present in the proportion of about one part by weight of trivalent chromium to about five parts by weight of hexavalent chromium, the temperature of the bath being approximately -130" to 135 Fahrenheit.

7. The process of electroplating with chromium, which comprises maintaining a bath substantially free from iron-and wherein chromic acid (010:) and sulphuric acid (S04) are pres Leeaeee I ent in a ratio of less than 40 parts, by Weight of chromic acid (CrOs) to one part by weight of sulphuric acid (S04) with the chromium present in the proportion of about one part by weight of trivalent chromium, to about five parts by weight of hexavalent chromium, the temperature of the bath being approximately 130 130135 Fahrenheit, and the current density being in the neighborhood of 600 to 900 amperes per square foot.

8. The process of electroplating with chromium, which comprises maintaining a bath substantially free from iron and wherein chromic acid (Ci-0a) and sulphuric acid (S04) are present in a ratio of about 18.5 parts by weight of chromic acid (CI'O3) to one part by weight of sulphuric acid (S04) with the chromium present in the proportion of about one part by weight of trivalent chromium to about five parts by weight of hexavalent chromium, the temperature of the bath being approximately 130 to 135 Fahrem belt, and the current density being in the neighborhood of 750 to 900 amperes per square foot.

' 9. The process of electroplating with chromium, which comprises maintaining a bath substantially free from iron and wherein chromic acid. (ClO3) is present in the proportion of about 240 grams per liter, and sulphuric acid (S04) is present in the proportion of about 13 grams per liter, chromium being present in the proportion of about one part by weight of trivalent chromium to about five parts by weight of hexavalent chromium, the temperature of the bath being approximately 130 to 135 Fahrenheit, the current density being in the neighborhood of 600 to 900 amperes per square foot. 

